The present invention relates to an objective optical system for an optical pick-up that writes information data onto an optical disc and/or reads information data from an optical disc.
There are several types of optical discs such as a CD (compact disc), a CD-R (CD recordable) or a DVD (digital versatile disc). The recording density of a DVD is larger than that of a CD or a CD-R. Further, an optical disc that has a still larger recording density has been developed. When the recording density becomes larger, a diameter of a beam spot formed on an optical disc is required to be smaller. Since the beam spot diameter is in inverse proportion to a numerical aperture (NA) and is in proportion to a wavelength of a light beam, it is necessary to increase the NA of an objective lens or to shorten the wavelength of the light beam in order to decrease the beam spot diameter.
The large NA requires the large effective diameter for the objective lens. However, when the objective lens is designed as a single lens element, the radius of curvature of at least one lens surface becomes significantly small, which results in too large of a center thickness to keep an appropriate edge thickness. Therefore, when a large NA objective optical system is employed to decrease the beam spot diameter, the size and weight of the objective optical system becomes too large to make the optical pick-up compact and light.
On the other hand, when a working wavelength becomes shorter, a wavelength dependence of the refractive index of the lens material increases. For instance, the wavelength dependence of the refractive index of lens material that is generally used to make the objective lens is xe2x88x923xc3x9710xe2x88x925 nmxe2x88x921 at the wavelength in the vicinity of 650 nm, while the wavelength dependence of the same material is xe2x88x9215xc3x9710xe2x88x925 nmxe2x88x921 at the wavelength in the vicinity of 400 nm. A semiconductor laser, which is employed as a light source of an optical pick-up, has a tolerance of an emission wavelength due to an individual difference, and an emission wavelength varies due to temperature change or the like. Therefore, the objective optical system is required to reduce the variation of the aberration due to the change of the wavelength. Particularly, when the working wavelength is smaller than F-line, since a focal depth becomes smaller in addition to the increasing of the wavelength dependence, it is important to correct the chromatic aberration.
A conventional method for correcting chromatic aberration is to combine a plurality of glass lenses whose Abbe numbers are different from each other. Further, Japanese patent provisional publication No. Hei 11-337818 discloses an objective lens that is a combination of a refractive lens and a diffractive lens structure formed on one surface of the refractive lens for correcting chromatic aberration. This publication teaches that a focal length fD of the diffractive lens structure and a focal length f of the entire objective optical system should satisfy a condition 40 less than fD/f. Further, it is assumed that the objective lens is made of plastic through an injection molding method in order to increase accuracy and to reduce cost.
However, since a transmittance of optical glass, particularly of high dispersion optical glass is significantly low at the wavelength shorter than F-line, a loss of light amount becomes too large when a plurality of lenses are used for correcting chromatic aberration. On the other hand, when the condition disclosed in the Japanese patent provisional publication No. Hei 11-337818 is applied to an objective optical system used at the wavelength shorter than F-line, the chromatic aberration cannot be adequately corrected even if any lens material is employed.
It is therefore an object of the present invention to provide an objective optical system that is capable of correcting chromatic aberration at a wavelength shorter than F-line.
For the above object, according to the first aspect of the present invention, there is provided an improved objective optical system that includes a refractive lens on which the diffractive lens structure is formed and satisfies the following condition (1);
1/(xcexd3xc3x97xcexxc3x9710xe2x88x926) less than 0.0045xe2x80x83xe2x80x83(1)
where xcexd is an Abbe number and xcex is a working wavelength (unit: nm).
At least one surface of the refractive lens is formed as an aspherical surface whose radius of curvature increases as a height from the optical axis becomes large. The diffractive lens structure has a plurality of concentric ring-shaped steps to correct chromatic aberration caused by the refractive lens.
In general, dispersion of lens material is represented by the Abbe number xcexd that is calculated from refractive indexes at C-line (656 nm), F-line (486 nm) and d-line (588 nm). Since the Abbe number increases as the dispersion becomes smaller, the lens material whose Abbe number is large is advantageous to prevent occurrence of chromatic aberration.
The dispersion, which is variation of the refractive index due to wavelength change, tends to become large as the wavelength becomes shorter. A difference between the dispersions of two lens materials is nearly equal to the cube of the difference between the Abbe numbers thereof at the wavelength shorter than F-line. Therefore, the cube of the Abbe number should be used as an index of the chromatic aberration at the wavelength shorter than F-line instead of the Abbe number.
On the other hand, the maximum permissible level of the chromatic aberration at a predetermined wavelength is proportional to the wavelength. Because the focal depth DOE, which is considered as an index of the maximum permissible level of the chromatic aberration, is also proportional to the wavelength as represented by DOF=kxcex/NA2 (k is a constant of proportionality).
Therefore, since the maximum permissible level of the chromatic aberration is represented by the reciprocal of the cube of the Abbe number 1/xcexd3, 1/xcexd3xcex less than K (K is a constant of proportionality) should be held to control the chromatic aberration in the wavelength shorter than F-line.
Thus, the product of the reciprocal of the wavelength xcex and the reciprocal of xcexd3 should be smaller than a predetermined value in order to reduce the chromatic aberration caused by the refractive lens at the wavelength shorter than F-line. The condition (1) defines the upper limit of the product.
With the above construction, the wavelength dependence of the refractive index at the working wavelength can be kept small, which reduces the chromatic aberration caused by the refractive lens. As a result, the diffractive lens structure adequately corrects the chromatic aberration.
The refractive lens on which the diffractive lens structure is formed is preferably made of glass. Since deformation and variation of refractive index of glass due to temperature change is smaller than that of plastic, the diffractive lens structure can be designed without consideration of the deformation and the variation of the refractive index, which allows the diffractive lens structure to be designed to adequately correct the chromatic aberration.
Further, it is preferable to satisfy the following conditions (2) and (3) when the objective optical system is applied to an optical disc whose information layer is covered by a transparent cover layer. The following conditions (3) and (4) are preferably satisfied when the objective optical system is applied to an optical disc whose information layer is not covered by a cover layer.
xe2x88x920.015 less than [xcex94nLxc2x7fDxc2x7f/{(nLxe2x88x921)xc2x7(fDxe2x88x92f)}xe2x88x92xcex94ndxc2x7td/nd2]xc2x7fD(fxc2x7NA/uhxe2x80x94d)2/f less than xe2x88x920.007 xe2x80x83xe2x80x83(2)
xe2x88x920.3 less than xcfx864/xcfx862 less than 0.3 xe2x80x83xe2x80x83(3)
xe2x88x920.015 less than [xcex94nLxc2x7fDxc2x7f/{(nLxe2x88x921)xc2x7(fDxe2x88x92f)}]xc2x7fD(fxc2x7NA/uhxe2x80x94d)2/f less than xe2x88x920.007xe2x80x83xe2x80x83(4)
where
xcex94nL is the rate of change of the refractive index of the refractive lens represented by the following equation,
xe2x80x83xcex94nL=(nL+1xe2x88x92nLxe2x88x921)/2
nL+1 is the refractive index of the refractive lens at the wavelength (xcex+1) nm;
nLxe2x88x921 is the refractive index of the refractive lens at the wavelength (xcexxe2x88x921) nm;
nL is the refractive index of the refractive lens at the working wavelength xcex nm;
fD Is the focal length of the diffractive lens structure represented by the following equation.
fD=xe2x88x92{1/(2P2xc3x97mxc3x97xcex)};
P1 is a coefficient of i-th order when the additional optical path length xcfx86(h) added by the diffractive lens structure is expressed by the following equation,
xcfx86(h)=(P0+P2h2+P4h4+P6h6+ . . . )xc3x97mxc3x97xcex;
h is a height from the optical axis;
m is a diffraction order;
f is the focal length of the objective optical system;
xcex94nd is the rate of change of the refractive index of the cover layer represented by the following equation,
xcex94nd=(nd+1xe2x88x92ndxe2x88x921)/2
nd+1 is the refractive index of the cover layer at the wavelength (xcex+1) nm;
ndxe2x88x921 is the refractive index of the cover layer at the wavelength (xcexxe2x88x921) nm;
nd is the refractive index of the cover layer at the working wavelength xcex nm;
td is a thickness of the cover layer;
NA is a numerical aperture of the objective optical system;
uh_d is an effective radius of the area within which the diffractive lens structure is formed; and
xcfx862 and xcfx864 are optical path differences represented by the following equations, respectively,
xcfx862=P2xc2x7uhxe2x80x94d2xc3x97mxc3x97xcex,
xcfx864=P4xc2x7uhxe2x80x94d4xc3x97mxc3x97xcex.
According to the second aspect of the present inventlon, there is provided an improved objective optical system that includes a single glass refractive lens and a chromatic aberration correcting element made of plastic on which a diffractive lens structure is formed and the refractive lens satisfies the above-mentioned condition (1).
At least one surface of the refractive lens is formed as an aspherical surface whose radius of curvature increases as a height from the optical axis becomes large. The diffractive lens structure has a plurality of concentric ring-shaped steps to correct chromatic aberration caused by the refractive lens.
With the above construction, the chromatic aberration caused by the refractive lens can be kept small because the refractive lens satisfies the condition (1). As a result, the diffractive lens structure formed on the chromatic aberration correcting element adequately corrects the chromatic aberration.
It is preferable that the chromatic aberration correcting element has no power at the working wavelength xcex and the following condition (5) is satisfied;
|xcex94nC/xcex94nL| greater than 2xe2x80x83xe2x80x83(5)
where
xcex94nC is the rate of change of the refractive index of the chromatic aberration correcting element represented by the following equation,
xcex94nC=(nC+1xe2x88x92nCxe2x88x921)/2
nC+1 is the refractive index of the chromatic aberration correcting element at the wavelength (xcex+1) nm; and
nCxe2x88x921 is the refractive index of the chromatic aberration correcting element at the wavelength (xcexxe2x88x921) nm.
Further, when the following condition (6) is satisfied, the number of the ring-shaped steps can be reduced, which eases the manufacturing of the chromatic aberration correcting element and increases the diffraction efficiency.
|xcex94nC/xcex94nL| greater than 4xe2x80x83xe2x80x83(6)
Since the diffractive lens structure is not formed on the refractive lens, it is unnecessary to take moldability and transforming ability into account when the refractive lens is designed, which allows the refractive lens to be made of lens material whose wavelength dependence of the refractive index is small. The transforming ability is an index to measure a transformation of a pattern formed on a molding die to a molded replica.
On the other hand, it is known that a value corresponding to Abbe number for a diffractive lens structure is equal to xe2x88x923.453. The negative sign of the value reflects the opposite sense of the dispersion when compared with that of optical glass or optical plastic, and its low magnitude is an indication of the large dispersion. That is, the chromatic aberration caused by the positive refractive lens can be corrected by means of the diffractive lens structure having small positive power. When the chromatic aberration correcting element has no power as described above, the positive diffractive power of the diffractive lens structure and the negative refractive power of the chromatic aberration correcting element as a refractive element are counterbalanced at the working wavelength xcex. That is, the negative refractive function of the chromatic aberration correcting element also corrects the chromatic aberration caused by the positive refractive lens. As a result, when the chromatic aberration correcting element is made of material whose wavelength dependence of the refractive index is large, the chromatic aberration correcting function required for the diffractive lens structure is reduced, which reduces the number of the ring-shaped steps, increasing the moldability and the transforming ability.
Further, it is preferable that the refractive lens is located between the chromatic aberration correcting element and the optical disc to keep a sufficient back focus (a working distance). The refractive lens is preferably arranged such that the surface having larger power faces the chromatic aberration correcting element.
It is preferable to satisfy the following condition (7) when the objective optical system is applied to an optical disc whose information layer is covered by a transparent cover layer. The following condition (8) is preferably satisfied when the objective optical system is applied to an optical disc whose information layer is not covered by a cover layer.
0.0001 less than [{xe2x88x922xc2x7P2xc2x7mxc2x7(1.0xc3x9710xe2x88x926)f+1}xe2x88x921xe2x88x92{xcex94nL/(nLxe2x88x921)}xe2x88x921]xc2x7f+xcex94ndxc2x7td/nd2 less than 0.0003xe2x80x83xe2x80x83(7)
0.0001 less than [{xe2x88x922xc2x7P2xc2x7mxc2x7(1.0xc3x9710xe2x88x926)f+1}xe2x88x921xe2x88x92{xcex94nL/(nLxe2x88x921)}xe2x88x921]xc2x7f less than 0.0003xe2x80x83xe2x80x83(8)